Thousands attended Apple's Worldwide Developers Conference at the Convention Center in San Jose on Monday.

CEO Tim Cook took the stage first and welcomed everyone to San Jose. "It's great to be in San Jose, the heart of the Silicon Valley and just down the street from Apple and our new campus. It's been 15 years since we held a developers conference in San Jose, and a lot has changed," he said.

Developers were happy to hear about the latest features for the Apple Watch and Mac operating systems.

Cook also welcomed the youngest developer, a 10-year-old from Australia and the oldest developer, an 82-year-old woman from Japan. They are two of 5,300 developers here Monday, the most ever for this event.

San Jose hosted Apple's Worldwide Developers conference for the first time since 2002. "It was my dream from so many years ago to finally come to an Apple keynote or a WWDC and it finally came true," Abdul Kadi said.

Security presence was everywhere. San Jose police blocked traffic from driving on west San Carlos Street in front of the crowds and convention center.

An officer told ABC7 News they started planning for this event three months ago, and terrorism is something they are taking into consideration.

Attendees have to go through metal detectors and detection dogs were omnipresent in the crowds.

San Jose leaders are happy this event moved from San Francisco to the South Bay, filling hotels and restaurants. However, some attendees are a little disappointed it's not being held in San Francisco. "It's okay. It's not San Francisco, but it's okay. It will still be good. We'll still learn the same things," Medco Specialty Distributors spokesperson Colleen Hendry said.

Expect to see the crowds downtown all week because the conference runs through Friday.